1. Field of the Invention
The present invention relates to a portable, combustion-engined tool, in particular, a setting tool for driving in fastening elements.
2. Description of the Prior Art
A tool of a type discussed above is disclosed in European Publication EP 0 818 281A2. The known tool includes a combustion chamber, a piston adjoining the combustion chamber, a pressure build-up in the combustion chamber providing for movement of the piston from its initial position, in which it adjoins the combustion chamber, away from the combustion chamber, and means for retaining the piston substantially in its initial position during build-up of pressure in the combustion chamber.
The device for retaining the piston is usually used to insure that the piston would not be displaced from its initial position at its upper dead point in the period between two setting processes and to retain the piston with a predetermined retaining force until a relatively high pressure is built-up in the combustion chamber. The retaining device should provide a high power yield to prevent the displacement of the piston already at the beginning of the pressure build-up, which leads to reduction of the flame front speed which, in turn, leads to transmission only of a small portion of the energy to the piston.
Accordingly, an object of the present invention is to provide in a tool of the type described above means for adjusting the amount of energy which is generated in the combustion chamber and is transmitted to the piston.
This and other objects of the present invention, which will become apparent hereinafter, are achieve by changing the braking force applied to the piston. By adjusting the braking force, the force, which retains the piston in its initial position, can be adjusted within certain limits which, in turn, influence the amount of energy consumed by the piston. By appropriate constructive measures, the braking force acting on the piston can be so selected that maximum of the generated energy is transmitted to the piston. Thereby a better effectiveness of the tool is achieved.
Generally, the braking device can act on different portion of the piston, e.g., it can act on the piston plate or the piston rod. The braking effect can be achieved with different means: electromagnetic, pneumatic, hydraulic, mechanical, and the like. Further, the braking force applied by the braking device can also vary dependent on the position of the piston. After the force retaining the piston in its initial position, which is predetermined by the braking force, has been overcome, with the piston moving away from its initial position, the braking force, upon further displacement of the piston in the operating direction, can be increased or changed in any other desirable way. Upon return of the piston to its initial position the braking force can, e.g., be minimized or be removed altogether to provide for a complete return of the piston to its initial position.
According to one preferred embodiment of the present invention, the braking device is provided in the region of the piston rod, which only slightly increases the dimensions of the entire structure so that a compact tool is obtained. The braking device can have only one braking member biased against the piston rod under an action of a compression spring which, according to further development of the invention, can be formed as a circular spring surrounding the piston rod. When the biasing spring is formed as a circular spring, a plurality of braking members can be arranged between the piston rod and the spring which produces a better braking effect. As a braking member, a roll member, a ball, a cylinder, and the like can be used. Such a roll member is held by a circular spring in a cage-like manner and rolls with respect to the piston rod when the piston rod moves through the braking device. The use of roll members permits to obtain a braking device with minimum wear.
In accordance with a further embodiment of the braking device according to the present invention, there is provided an actuation member having an adjustable actuation force and acting on the spring. By actuating the actuation member, the braking force acting on the piston can be change in a simple manner.
When the braking member biasing spring is formed as a circular spring, the actuation member can be formed as a spring stirrup having two legs between which the circular spring, together with the piston rod is received. Dependent on movement of the stirrup base toward or away from the piston rod, the legs would expand to a greater or lesser degree, applying, respectively, a smaller or greater force to the spring which, in turn, applies a smaller or greater biasing force to the braking member. The displacement of the stirrup base changes, thus, the braking force applied by the braking member to the piston rod.
The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to its construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiments, when read with reference to the accompanying drawings.